Adds checks to coderd/healthcheck/derphealth for STUN issues:
- Alerts if there is not least one healthy STUN server,
- Alerts if we see variable port mapping.
Relates to #8965
- Added error codes for separate code paths in health checks
- Prefixed errors and warnings with error code prefixes
- Added a docs page with details on each code, cause and solution
Co-authored-by: Muhammad Atif Ali <atif@coder.com>